This invention relates to methods of manufacturing all-ceramic dental restorations and more specifically to methods of manufacturing dental restorations using CAD/CAM techniques.
The fabrication of current all-ceramic dental restorations often requires extensive labor and time and the proficiency of highly skilled technicians. Many state-of-the-art dental restorations reveal a sense of artistry that can typically only be achieved manually or xe2x80x9cby hand.xe2x80x9d While aesthetics are preserved with this process, microstructural inhomogeneities may appear, affecting strength and reliability. The industry has attempted to automate this process by, for example, pressing crowns. Although pressable crowns reduce the time required to produce a crown, about two hours of concerted effort is necessary to complete a crown. Pressed crowns may also suffer from similar strength and reliability problems typical of xe2x80x9chand madexe2x80x9d crowns.
Computer assisted design/computer assisted milling (CAD/CAM) processes and equipment have been recently introduced into the dental industry. In these processes, a three-dimensional image of a stump of a tooth is created along with the teeth surrounding the stump in an effort to create a dental restoration which is to be placed over the stump. This image is displayed on a computer screen. Based on the stump and surrounding teeth, the dental technician may then select a tooth from a plurality of tooth forms stored in the computer to best fit the stump. The selected tooth is projected onto the stump until an optimum positioning and fit of the dental restoration is achieved. The digital data concerning the dental restoration thus formed are supplied to a numerically controlled milling machine operating in three dimensions. The milling machine cuts a blank of metal or porcelain material into the dental restoration design based on the data supplied.
U.S. Pat. No. 4,663,720 to Duret and commonly assigned U.S. Pat. No. 5,775,912 to Panzera et al. each teach CAD/CAM systems and materials which are designed to reduce labor and increase reliability and are herein incorporated by reference. U.S. Pat. No. 5,775,912 is directed to a method of making a dental restoration using soft-sintered porcelain pellets. The method requires the step of investing the tooth structure with an investment refractory material prior to fusing and fully densifying because the glass-ceramic will begin to flow during this step. The investment refractory material provides a mold to maintain the shape of the glass-ceramic during sintering.
U.S. Pat. No. 5,910,273 to Thiel teaches a process for the manufacture of dental materials using CAD/CAM methods wherein a porosity-sintered blank is milled to a desired shape. In order to densify the material, it must be infiltrated with a glass material.
CAD/CAM and copy milling systems designed for the dental industry by Vita Celay, Siemens and Nobelpharma have also been shown to reduce labor. However, some of the materials used in these systems have been shown to be weak or unaesthetic. Moreover, it has been observed, that the use of fully fused dental porcelain pellets wear down cutting and milling tools and significantly slow down the process of dental restoration fabrication. The milling of fully fused dental porcelains may result in excessive chipping and flaking, thus affecting the precision of the milling operation and ultimately the fit between the restoration and the patient""s natural teeth.
There is a need to provide materials for use in CAD/CAM operations that are strong and aesthetically pleasing. It is desirable to provide materials for use in CAD/CAM operations that reduce wear of cutting tools on milling machines.
These and other objects and advantages are accomplished by the material of the present invention comprising a soft-sintered or non-sintered (held together by binders) block of ceramic material. The ceramic material consists of fairly uniform particles thoroughly dispersed to be essentially free of agglomerates such that it will sinter predictably and isotropically without appreciable distortion. The material may be aluminum oxide, partially stabilized zirconium oxide, mullite, any suitable oxide ceramic or glass-ceramic material which may be sintered to high strength (i.e., greater than 250 MPa, and preferably greater than 400 MPa) or mixtures thereof The ceramic materials are used to manufacture dental materials including, but not limited to, orthodontic appliances, bridges, space maintainers, tooth replacement appliances, splints, crowns, partial crowns, dentures, posts, teeth, jackets, inlays, onlays, facings, veneers, facets, implants, abutments, cylinders, and connectors.
In one embodiment of the method of the invention, ceramic powders are combined with a binder and pressed into blocks or similar shapes to form green bodies. The ceramic powders consist of fairly uniform particles thoroughly dispersed to be essentially free of agglomerates such that they will sinter predictably and isotropically without appreciable distortion. The green bodies are milled to a desired shape which is oversized to account for anticipated shrinkage during the sintering stage, and sintered to a final density rendering a high strength dental restorative material. The xe2x80x9cplastic statexe2x80x9d of the green bodies allows for easy milling into complicated shapes. The material may be aluminum oxide, partially stabilized zirconium oxide, mixtures of the two, mullite or any suitable oxide or glass-ceramic material that may be sintered to high strength (i.e., greater than 250 MPa, and preferably greater than 400 MPa) or mixtures thereof
In another embodiment of the method of the invention, ceramic precursor powders are combined with a binder and pressed into blocks or similar shapes to form green bodies. The ceramic powders consist of fairly uniform particles thoroughly dispersed to be essentially free of agglomerates such that it will sinter predictably and isotropically without appreciable distortion. The green bodies are soft-sintered to a bisque density that is between about fifty percent (50%) and about eighty-five percent (85%) of the final density. The soft-sintered blocks are then milled to a desired shape, which is oversized to account for anticipated shrinkage during the sintering stage, and sintered to a final density rendering a high strength dental restorative material. The soft-sintered state of the blocks allows for easy milling into complex or elaborate shapes. The material may be aluminum oxide, partially stabilized zirconium oxide, mixtures of the two, mullite or any suitable oxide or glass-ceramic that may be sintered to high strength (i.e., greater than 250 MPa, and preferably greater than 400 MPa) and mixtures thereof
In yet another embodiment of the method of the invention, a computer assisted milling machine is used to mill a die that is an oversize copy of a patient""s tooth or teeth. Thereafter, a ceramic powder with or without binder is applied onto the machined die and the powder-die pair is isostatically pressed to allow the powder to form a uniform mass. Powders can be applied by slip-casting or gel-casting. The amount of die oversized is equivalent to the amount of shrinkage expected for the ceramic powder. The isostatically pressed mass will shrink uniformly when sintered at a sufficiently high temperature to form a dental coping. The die material shrinks considerably more than the coping or melts or burns out and does not interfere with it. The ceramic coping material consists of fairly uniform particles thoroughly dispersed to be essentially free of agglomerates such that it will sinter predictably and isotropically without appreciable distortion. The material may be aluminum oxide, partially stabilized zirconium oxide, mullite or any suitable oxide or glass-ceramic that may be sintered to high strength (i.e., greater than 250 MPa, and preferably greater than 400 MPa), or mixtures thereof